1. Field of the Invention
The present invention relates to an electrophotographic apparatus. More particularly, the present invention relates to a tandem-type color electrophotographic apparatus including image forming units using toner of a plurality of colors.
2. Description of the Related Art
Nowadays, organic photosensitive members are widely used as photosensitive drums, which are image bearing members, of electrophotographic apparatuses for cost and productivity reasons. An organic photosensitive member includes a photosensitive layer (organic photosensitive layer), which is made of an organic material (charge generation material or a charge transfer material) and formed on a supporting member. As the organic photosensitive member, a photosensitive drum (hereinafter referred to as a drum) having a stacked type photosensitive layer is in the mainstream. This is because it offers high sensitivity and allows diverse material designs. The stacked type photosensitive layer includes a charge generation layer including the charge generation material and a charge transfer layer including the charge transfer material.
Generally, various layers are provided between the supporting member and the photosensitive layer. The layers are helpful in enhancing coatability of the surface of the supporting member, coating performance of the photosensitive layer, adhesion of the supporting member and the photosensitive layer, protection performance of the photosensitive layer from electrical damage, charging property, and charge injection property of charges from the supporting member to the photosensitive layer.
For example, a conductive layer and a middle layer are provided between the photosensitive layer and the supporting member. The conductive layer coats the surface of the conductive supporting member. The middle layer can act as a barrier for electrically preventing the charge injection from the conductive layer to the photosensitive layer. By providing these layers, a drum with stable quality and manufacturing can be obtained. As a binding resin of the transportation layer of the drum, a polycarbonate resin and a polyallylate resin are widely used. The polyallylate resin enhances the mechanical strength.
Further, a contact charge roller is widely used as a charging device that charges the drum so that the drum has a predetermined polarity and electric potential since it is inexpensive and allows ozoneless configuration. The contact charge roller has a layer of an elastic rubber material on a cored bar. The contact charge roller is pressed against the drum at a predetermined pressure and rotates according to the rotation of the drum. The electric discharge occurs according to an application of a predetermined bias to the cored bar. A predetermined surface potential of the drum is obtained by this electric discharge.
The electrophotographic apparatus also includes a development device. The development device, which is used for developing an electrostatic latent image by developer, includes a developer container that contains toner as the developer, a developing roller that conveys the toner to the drum, and a developer restricting member which applies charge to the toner so that a thin layer is uniformly formed on the surface of the developing roller.
As toner for a full-color electrophotographic apparatus, nonmagnetic one-component toner is generally used. Further, as the developing method, a method called contact developing method is widely used since it allows inexpensive and small configuration. The contact developing method is used for transferring toner on the developing roller to the drum by causing the developing roller, having a thin layer of toner, to contact the drum.
Further, the electrophotographic apparatus includes a cleaning device which removes untransferred developer from the drum. The cleaning device is configured of a cleaning blade formed by an elastic rubber material such as urethane rubber and a waste toner container. The removed toner is collected in the waste container. A method called counter contact cleaning method is widely used as the cleaning method. According to this method, the cleaning blade is pressed against the drum at a predetermined pressure and the toner that remains on the drum surface is physically scraped and collected by the cleaning blade.
As the full-color electrophotographic apparatus, a tandem type apparatus which allows high speed printing is widely used. The tandem type apparatus includes a plurality of drums each of which corresponds to each toner color. Normally, a full-color electrophotographic apparatus uses toner of four colors, which are yellow, magenta, cyan, and black. A color image can be reproduced by superimposing each toner.
Regarding the tandem type apparatus, image forming units for respective colors are provided along the intermediate transfer member. Each of the image forming units includes a drum, a charging device, a development device, and a cleaning device. The image forming unit of each color is hereinafter referred to as an image forming station. As the image forming process, toner developed on the drum at each image forming station is primary transferred onto the intermediate transfer member at each image forming station. Then, the toner image is simultaneously secondary-transferred onto a print medium such as paper.
Generally, a full-color electrophotographic apparatus is capable of monochromatic printing using black toner in a mode called monochrome mode. When the full-color electrophotographic apparatus is in the monochrome mode, it is desirable not to use the drum and the development device of each image forming station other than the black image forming station so as to extend the life of such devices.
However, in order to prevent upsizing of the apparatus and reduce the cost due to complexity of devices (e.g., drive device), some full-color electrophotographic apparatuses have the drums of all colors rotate in the monochrome mode although they do not drive the developing rollers of the image forming stations of the colors other than black. Further, some apparatuses perform exposure of drums corresponding to image forming stations of colors other than black so that the static charge is eliminated from the drums and the untransferred black toner is not collected at the image forming stations of colors other than black when the drums of all colors rotate.
Further, there are cartridge-type image forming apparatuses each of which detachably mounts a process cartridge including the charging device, the drum, the development device, and the cleaning device on the apparatus main body of the image forming apparatus. With this system, the maintenance of the image forming apparatus can be performed by a user without help of a service engineer. For example, if the toner runs out or the drum reaches the end of its life, the user of the image forming apparatus can replace the cartridge to form an image again.
Regarding the cartridge-type image forming apparatus, when consumables such as the drum or the developer reach the end of their life or the end of their life is nearing, it has to be informed to the user so that the cartridge can be replaced by a new one at suitable timing by a user. The drum life is generally determined by the amount of film thickness of the surface layer, which is a charge transfer layer including a charge transfer material, which wears over time. The surface layer is hereinafter referred to as a CT layer.
As described above, during the image forming process, an electrical/mechanical external force is applied to the drum, for example, by the discharge process, sliding friction by the developing roller and the intermediate transfer member, and scraping by the cleaning blade. As a result, the CT layer wears. Under such circumstances, there are proposed various methods for determining the drum life before image unevenness and fogging becomes noticeable by estimating the wear amount of the CT layer.
Among such proposes, Japanese Patent Application Laid-Open No. 2001-356655 discusses a life determining unit which determines the life of a drum by comparing a value obtained by integrating the application time of the charge unit or the application time of the development unit for the drum, with life information of the drum.
FIG. 5 illustrates the arrangement of the drum, the charge roller, the developing roller, and the cleaning blade of a conventional publicly-known contact development system in the longitudinal direction of the drum. In FIG. 5, a photosensitive drum 100 as an image bearing member, a charge roller (charging member) 201 as a charging device, a developing roller 401 as a developer bearing member, a cleaning blade rubber unit (hereinafter referred to as a cleaning blade) 601 as a cleaning member, toner seals 210 as toner seal members are arranged in the contact development system.
A toner seal 210 is provided at each end portion of the developing roller 401, between the frame of the development device and the developing roller 401, in order to prevent the toner in the developer container from being scattered. Each end portion of the developing roller 401 is pressed by the toner seal 210. Accordingly, the toner in the developer container is blocked, and scattering of toner at both end portions of the developing roller 401 is prevented.
An area of the developing roller 401 between the contact positions of the toner seals 210 is a toner bearing area (developer bearing area) 220 of the developing roller 401. The areas of the developing roller 401 on the outer sides of the toner bearing area 220 are toner non-bearing areas (developer non-bearing areas) 230 of the developing roller 401.
The surface of the drum 100 which the toner bearing area 220 of the developing roller 401 contacts is referred to as an image forming area (first area) 120. The surface areas of the drum 100 on the outer sides of the first area 120 are referred to as non-image forming areas (second areas) 130.
In order to make the process cartridge smaller in size, in many cases, the end portions of the developing roller 401 are configured so as to be within the area of the cleaning blade 601 in the longitudinal direction. Further, as a portion “d” in FIG. 5 indicates, the end portions of the developing roller 401 may be arranged within a range of 1 to 2 millimeters from the end portions of the charge roller 201. In this configuration, the end face of the charge roller 201 is close to the end face of the developing roller 401. Further, in many cases, both end faces thereof are arranged within the area of the cleaning blade 601. In other words, the end portions of the charge roller 201 and the end portions of the developing roller 401 are within the scraping area of the cleaning blade 601.
In the configuration illustrated in FIG. 5, the wear amount of the CT layer of the drum 100 is not uniform in the longitudinal direction of the drum 100. Especially, the wear amount of the CT layer in the first area 120 to which the toner bearing area 220 corresponds and in the second area 130 to which the toner non-bearing area 230 corresponds are different in the longitudinal direction of the developing roller 401.
The difference mainly occurs due to the difference in the characteristics of the end portions of the developing roller 401 and the end portions of the charge roller 201. Generally, the peripheral speed of the developing roller 401 is faster than the drum 100. Accordingly, the friction caused by the contact and friction of the developing roller 401 wears out the CT layer by mechanical stress. Since the stress that occurs when the developing roller 401 contacts the drum 100 is changed depending on the presence of the toner, the wear amount of the CT layer is different between the first area 120 and the second area 130. In other words, since the developing roller 401 directly contacts the CT layer in the second area 130 where toner is not present, the wear amount of the CT layer in the second area 130 is greater than the wear amount in the first area 120 where toner is present.
Further, at the end portions of the charge roller 201, the electric discharge from the end face of the charge roller 201 is added to the electric discharge from the periphery of the charge roller 201. Thus, the wear amount of the CT layer of the drum 100 corresponding to the end portions of the charge roller 201 is greater than the wear amount of the CT layer corresponding to the portions other than the end portions of the charge roller 201.
Further, as the portion “d” in FIG. 5 indicates, the end portion of the developing roller 401 may be in a range of approximately 2 mm from the end portion of the charge roller 201 and, further, the end portions of both the developing roller 401 and the charge roller 201 may be in the scraping region of the cleaning blade 601. With such configuration, the wear amount of the CT layer at the end portions of the second area 130 where the toner is not present is furthermore increased compared to the first area 120 where the toner is present.
If the wear at the end portions progresses, all the CT layer of the portions corresponding to the end portions may wear out and the conductive layer of the drum 100 may be exposed. In this case, since the periphery of the developing roller 401 or the charge roller 201 directly contacts the conductive layer of the drum 100, leaking of the bias applied to the developing roller 401 or the charge roller 201 to the conductive layer may occur. This leads to fogging due to defective charge or defective image due to defective development. Thus, it is necessary to inform the user that the drum is to reach the end of its life before the CT layer of the drum end portions wears out and the conductive layer is exposed.
If the CT layer at the drum end portions significantly wears out, the life of the drum 100 may be informed according to the wear of the CT layer at the end portions of the second area 130 instead of the wear of the CT layer in the first area 120 which corresponds to the toner bearing area 220 of the developing roller 401.
Further, some image forming apparatuses in the monochrome mode expose the drum 100 of each of the image forming stations of colors other than black to light to remove the static charge while charging the drum 100 so that black toner that remains after the secondary transfer is not collected by the image forming stations other than the black image forming station.
In this case, since the developing roller 401 of each of the image forming stations of yellow, magenta, and cyan does not contact the drum 100 in the monochrome mode, the wear of the CT layer in the second area 130 is smaller in amount.
Under such circumstances, if the time for replacement of the drum 100 is informed based on the wear amount of the CT layer in the second area 130, since the amount of discharge in the first area 120 of the drum 100 is greater and thus the CT layer wears faster, the CT layer in the first area 120 may wear out before the user is informed of the wear.